Amorphous Calcium Phosphate as Bioactive Filler in Polymeric Dental Composites

As biocompatible and osteo-inductive precursor to biological apatite formation, amorphous calcium phosphate (ACP) resorbs at the rate that closely coincides with the rate of new bone formation and is more osteo-conductive than its crystalline counterpart. In addition, in the oral environment, ACP intrinsically provides a protracted supply of the remineralizing calcium and phosphate ions needed for regeneration of mineral lost to tooth decay. These features make ACP composites a strong remineralizing tool at the site of caries attack. Our group has been on the forefront of the research on bioactive, remineralizing, polymeric ACP-based dental materials for over two decades. This entry describes methods for filler, polymer, and composite fabrication and a battery of physicochemical and biological tests involved in evaluation of ACP-based restoratives. Also presented is our most recent design of ACP remineralizing composites with added antimicrobial capability that shows promise for extended dental and, potentially, wider biomedical applications

Effects of Protein-Coated Nanofibers on Conformation of Gingival Fibroblast Spheroids: Potential Utility for Connective Tissue Regeneration

Deep wounds in the gingiva caused bytrauma or surgery require a rapid and robust healing of connective tissues. Wepropose utilizing gas-brushed nanofibers coated with collagen and fibrin for that purpose. Our hypotheses are that protein-coated nanofibers will: (i) attract and mobilize cells in various spatial orientations, and (ii) regulate the expression levels ofspecific extracellular matrix (ECM)-associated proteins, determining the initial conformational nature ofdense and soft connective tissues. Gingival fibroblast monolayers and3D spheroids were cultured onECMsubstrate and covered with gas-blown poly-(DL-lactide-co-glycolide)(PLGA) nanofibers (uncoated/coated with collagen and fibrin). Cell attraction and rearrangement was followed byF-actin staining and confocal microscopy. Thicknesses ofthe cell layers, developed within the nanofibers, were quantified byImageJ software. The expression ofcollagen1α1 chain (Col1α1), fibronectin, and metalloproteinase 2 (MMP2) encoding genes was determined byquantitative reverse transcription analysis. Collagen- and fibrin- coated nanofibers induced cell migration toward fibers and supported cellular growth within the scaffolds. Both proteins affected the spatial rearrangement offibroblasts byfavoring packed cell clusters or intermittent cell spreading. These cell arrangements resembled the structural characteristic ofdense and soft connective tissues, respectively. Within three days ofincubation, fibroblast spheroids interacted with the fibers, and grew robustlybyincreasing their thickness compared to monolayers. While theECMkeycomponents, such as fibronectin andMMP2encoding genes, were expressed in both protein groups, Col1α1 was predominantlyexpressed in bundled fibroblasts grown on collagen fibers. This enhanced expression ofcollagen1 is typical for dense connective tissue. Based on results ofthis study, our gas-blown, collagen- and fibrin-coated PLGA nanofibers are viable candidates for engineering soft and dense connective tissues with the required structural characteristics and functions needed for wound healing applications. Rapid regeneration of these layers should enhance healing ofopen wounds in a harsh oral environment

Determination of leachable components from an experimental endodontic sealer by nuclear magnetic resonance (NMR) spectroscopy

Introduction. Studies of experimental composites based on amorphous calcium phosphate (ACP) intended for dental applications as remineralizing pit and fissure sealants, orthodontic adhesives and, most recently, endodontic sealers were mainly focused on improving their mechanical, esthetic and remineralizing capabilities. Similar to the vast majority of commercial composites, these experimental formulations might have clinical drawbacks due to incomplete conversion of monomers and leachability of monomers and degradation products, shrinkage and stress associated with polymerization, and microleakage typically occurring at the tooth/composite interfacial region. The aim of this study was to identify and quantify leachable components from the experimental amorphous calcium phosphate (ACP) endodontic sealer using 1H nuclear magnetic resonance (NMR) spectroscopy. Material and Methods. The light-cure resin [camphorquinone (CQ) + ethyl-4-N,N-dimethylamino benzoate (4EDMAB)] was formulated from urethane dimethacrylate (UDMA), poly(ethylene glycol)- extended urethane dimethacrylate (PEG-U), 2-hydroxyethyl methacrylate (HEMA) and methacryloyloxyethyl phthalate (MEP). The experimental sealer contained 40 mass % ground ACP and 60 mass % resin. The copolymer (unfiled resin) and ACP/UPHM composite specimens, after being dried to a constant mass, were extracted in butylated hydroxyl toluene (BHT; prevents polymerization of eluted components) containing acetone for 7 days at 23ºC with continuous magnetic stirring. Gravimetric changes were recorded for each specimen. After solvent evaporation, 1H NMR spectra were collected on the extractable portion of each specimen. Representative peaks for each component were used to calculate their relative portions in the extracts. By combining the gravimetric and NMR data, the overall loss of each monomer and its concentration in the extract was calculated. Results were statistically analyzed by ANOVA and multiple pair-wise comparisons (t-test). Results. No CQ could be detected in the extracts. However, 33.06 % and 24.66 % of the initially incorporated 4EDMleached from the copolymers and composites, respectively. 0.30-14.29% and 0.12-10.39% of each monomer content leached out from copolymers and composites, respectively. The apparent differences in the concentrations between copolymers and composites became marginal when composite data were normalized with respect to the initial amount of the resin. Conclusion. 1H NMR conveniently provides qualitative and quantitative information on leachables without the elaborate sample preparation and/or data interpretation. Leachability of the unrectaed monomers from the experimental ACP sealer is apparently controlled by the highly cross-linked resin network and unaffected by the incorporation of bioactive ACP into the resin. The maximum levels of leachables from our experimental composite were within or below the concentration ranges reported for the commercial counterparts

Fine-Tuning of Polymeric Resins and their Interfaces with Amorphous Calcium Phosphate. A Strategy for Designing Effective Remineralizing Dental Composites

For over a decade our group has been designing, preparing and evaluating bioactive, remineralizing composites based on amorphous calcium phosphate (ACP) fillers embedded in polymerized methacrylate resin matrices. In these studies a major focus has been on exploring structure-property relationships of the matrix phase of these composites on their anti-cariogenic potential. The main challenges were to gain a better understanding of polymer matrix/filler interfacial properties through controlling the surface properties of the fillers or through fine-tuning of the resin matrix. In this work, we describe the effect of chemical structure and composition of the resin matrices on some of the critical physicochemical properties of the copolymers and their ACP composites. Such structure-property studies are essential in formulating clinically effective products, and this knowledge base is likely to have strong impact on the future design of therapeutic materials, appropriate for mineral restoration in defective tooth structures

Bioactive Polymeric Materials for Tissue Repair

Bioactive polymeric materials based on calcium phosphates have tremendous appeal for hard tissue repair because of their well-documented biocompatibility. Amorphous calcium phosphate (ACP)-based ones additionally protect against unwanted demineralization and actively support regeneration of hard tissue minerals. Our group has been investigating the structure/composition/property relationships of ACP polymeric composites for the last two decades. Here, we present ACP’s dispersion in a polymer matrix and the fine-tuning of the resin affects the physicochemical, mechanical, and biological properties of ACP polymeric composites. These studies illustrate how the filler/resin interface and monomer/polymer molecular structure affect the material’s critical properties, such as ion release and mechanical strength. We also present evidence of the remineralization efficacy of ACP composites when exposed to accelerated acidic challenges representative of oral environment conditions. The utility of ACP has recently been extended to include airbrushing as a platform technology for fabrication of nanofiber scaffolds. These studies, focused on assessing the feasibility of incorporating ACP into various polymer fibers, also included the release kinetics of bioactive calcium and phosphate ions from nanofibers and evaluate the biorelevance of the polymeric ACP fiber networks. We also discuss the potential for future integration of the existing ACP scaffolds into therapeutic delivery systems used in the precision medicine field

Structure-Composition-Property Relationships in Polymeric Amorphous Calcium Phosphate-Based Dental Composites

Our studies of amorphous calcium phosphate (ACP)-based materials over the last decade have yielded bioactive polymeric composites capable of protecting teeth from demineralization or even regenerating lost tooth mineral. The anti-cariogenic/remineralizing potential of these ACP composites originates from their propensity, when exposed to the oral environment, to release in a sustained manner sufficient levels of mineral-forming calcium and phosphate ions to promote formation of stable apatitic tooth mineral. However, the less than optimal ACP filler/resin matrix cohesion, excessive polymerization shrinkage and water sorption of these experimental materials can adversely affect their physicochemical and mechanical properties, and, ultimately, limit their lifespan. This study demonstrates the effects of chemical structure and composition of the methacrylate monomers used to form the matrix phase of composites on degree of vinyl conversion (DVC) and water sorption of both copolymers and composites and the release of mineral ions from the composites. Modification of ACP surface via introducing cations and/or polymers ab initio during filler synthesis failed to yield mechanically improved composites. However, moderate improvement in composite’s mechanical stability without compromising its remineralization potential was achieved by silanization and/or milling of ACP filler. Using ethoxylated bisphenol A dimethacrylate or urethane dimethacrylate as base monomers and adding moderate amounts of hydrophilic 2-hydroxyethyl methacrylate or its isomer ethyl-α-hydroxymethacrylate appears to be a promising route to maximize the remineralizing ability of the filler while maintaining high DVC. Exploration of the structure/composition/property relationships of ACP fillers and polymer matrices is complex but essential for achieving a better understanding of the fundamental mechanisms that govern dissolution/re-precipitation of bioactive ACP fillers, and, ultimately, the suitability of the composites for clinical evaluation

Structure-Composition-Property Relationships in Polymeric Amorphous Calcium Phosphate-Based Dental Composites

Our studies of amorphous calcium phosphate (ACP)-based materials over the last decade have yielded bioactive polymeric composites capable of protecting teeth from demineralization or even regenerating lost tooth mineral. The anti-cariogenic/remineralizing potential of these ACP composites originates from their propensity, when exposed to the oral environment, to release in a sustained manner sufficient levels of mineral-forming calcium and phosphate ions to promote formation of stable apatitic tooth mineral. However, the less than optimal ACP filler/resin matrix cohesion, excessive polymerization shrinkage and water sorption of these experimental materials can adversely affect their physicochemical and mechanical properties, and, ultimately, limit their lifespan. This study demonstrates the effects of chemical structure and composition of the methacrylate monomers used to form the matrix phase of composites on degree of vinyl conversion (DVC) and water sorption of both copolymers and composites and the release of mineral ions from the composites. Modification of ACP surface via introducing cations and/or polymers ab initio during filler synthesis failed to yield mechanically improved composites. However, moderate improvement in composite’s mechanical stability without compromising its remineralization potential was achieved by silanization and/or milling of ACP filler. Using ethoxylated bisphenol A dimethacrylate or urethane dimethacrylate as base monomers and adding moderate amounts of hydrophilic 2-hydroxyethyl methacrylate or its isomer ethyl-α-hydroxymethacrylate appears to be a promising route to maximize the remineralizing ability of the filler while maintaining high DVC. Exploration of the structure/composition/property relationships of ACP fillers and polymer matrices is complex but essential for achieving a better understanding of the fundamental mechanisms that govern dissolution/re-precipitation of bioactive ACP fillers, and, ultimately, the suitability of the composites for clinical evaluation